The invention relates to transparent substrates, in particular glass substrates, which are provided with coatings composed of one or more thin films which have interference-scale thicknesses and are designed to give specific properties to the substrates which bear them, for example, thermal, optical or electrical properties.
The invention also relates to the use of these coated substrates, in particular to produce glazing, as well as to the method of obtaining them.
The coatings mentioned above therefore consist of stacks of films with varied chemical composition and properties. They are most often dielectric films, for example of the metal oxide or nitride or silicon oxide type, and/or conductive films, for example films of metal such as silver or doped metal oxide. For optical reasons, in many cases these coatings include films whose refractive index is to be carefully selected.
A type of coating, referred to as an anti-reflection coating, is thus known which usually consists of an alternating sequence of dielectric films with high and low refractive indices. Deposited on a transparent substrate, a coating of this type has the function of reducing the light reflection factor of this substrate, and therefore of increasing its light transmission factor. A substrate coated in this way will thus have an increase in its transmitted-light/reflected-light ratio, which improves the visibility of objects placed behind it. It can then be used in many applications, for example to protect a picture illuminated by a light placed behind the observer, or to constitute or form part of a shop window, in order more clearly to discern what is contained in the window, even when the interior lighting is low compared to the exterior lighting, or alternatively in interior furniture or as an anti-glare screen arranged in front of computer screens.
The performance of an anti-reflection coating can be measured or evaluated on the basis of various criteria. Firstly, of course, are the optical criteria. It may be considered that a "good" anti-reflection coating should be able to lower the light reflection factor of a standard clear glass substrate to a given value, for example 2%, or 1% and less. Similarly, it may be important for this coating to keep a satisfactory colorimetry for the substrate, in particular in reflection, for example an extremely neutral one which is very close to that of the bare substrate. Other criteria may also be taken into account depending on the application which is envisaged, in particular the chemical and/or mechanical durability of the coating, the cost of the materials which are used, the manufacturing time or the techniques to be used for manufacturing it.
In this type of anti-reflection stack, as in others, it must therefore be possible to manufacture films of material with low refractive index, for example with an index of less than 1.65.
Various materials currently meet this criterion. Mention may be made of magnesium fluoride MgF.sub.2, with an index of about 1.38, which can be deposited in the form of a thin film by a technique of the vacuum evaporation type. This is a technique which is reliable for deposits on small surfaces, for example surfaces of spectacles or lenses, but becomes expensive and complicated when deposition on larger surfaces is involved, for example on glazing.
Silicon oxide SiO.sub.2, with a refractive index of about 1.45, may also be mentioned. This material can be deposited using a cathodic sputtering technique, which technique is entirely suitable for deposits on large surfaces of the glazing type. However, it is not very easy to deposit SiO.sub.2 by reactive sputtering in the presence of oxygen. Further to the fact that it is necessary to dope the silicon target with another element, in particular boron or aluminium, the deposition rate of the SiO.sub.2 film when using this technique is low, and the deposition conditions are sometimes difficult to stabilize and control.